Dual-porosity hydromechanical modeling of swelling processes in Opalinus clay

Opalinus clay exhibits extremely low permeability together with pronounced swelling and self-sealing behavior upon hydration, making it a highly promising host rock for deep geological repositories. This study presents a dual-porosity hydromechanical model to simulate the permeability evolution during swelling. The coupled hydromechanical behavior of clay rocks is described by unsaturated flow using Richards’ equation and by a linear, saturation-dependent swelling deformation. The hydromechanical coupling between swelling and flow is realized via a strain-dependent permeability formulation. The hierarchical pore structure of clay rock is incorporated into the model by distinguishing between macro and micro pore domains, to which different van Genuchten parameters are assigned. The mathematical model is implemented in the open-source software OpenGeoSys. Experimental validation of the model is provided by flow-through multi-load swelling experiments. In these experiments, our samples were flowed through by applying a differential water pressure between the inflow at a central bore of the cylindrical samples, and the outflow at the mantle. At the same time, axial swelling strain was measured during stepwise mechanical unloading. The permeability evolution was determined by the measured outflow rate and pressure difference between the inflow and outflow. The model was calibrated by a dual-objective NSGA-II (Non-dominated Sorting Genetic Algorithm II) optimization, which simultaneously calibrated the strain and permeability evolution. The model effectively reproduces the observed swelling strain development at the different load stages, as well as short-term, unloading-induced permeability changes and a long-term self-sealing trend. These results highlight the capability of the proposed model to predict long-term hydro-mechanical evolution in clay rocks.